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Circuit and electromagnetism basics

A DC voltage E is applied through a resistor R to an uncharged capacitor. Which correctly describes the capacitor immediately after the voltage is applied?

  1. AThe voltage across it is E and no current flows
  2. BThe voltage across it is 0; it behaves like a short circuit and a current E/R flows
  3. CThe voltage across it is E/2, sharing the voltage equally with the resistor
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Answer: B. The voltage across it is 0; it behaves like a short circuit and a current E/R flows

A capacitor's voltage is set by its stored charge (V = Q/C), and with a finite current the charge cannot change instantly, so neither can the voltage. Immediately after the voltage is applied, the capacitor voltage is still 0, the capacitor behaves like a short circuit, and the current is E/R. The voltage then approaches E with time constant RC, and after a long time the current falls to 0 and the capacitor looks like an open circuit.

  • A. A voltage of E with no current is the state after a long time.
  • B. Correct. The voltage cannot change suddenly, so it starts at 0 and a current E/R flows.
  • C. The voltage is shared equally at one moment during charging, but not immediately after the voltage is applied.
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